Which motor drivers use autobauding in packet serial?
V1 motor drivers use autobauding. The list of V1 motor drivers is as follows:

SyRen 10

SyRen 25

Sabertooth 2X5

Sabertooth 2X10

Sabertooth 2X25 V1

V2 and newer motor drivers store their baud rate in an EEPROM setting (the factory default is 9600).

I have a V1 motor driver and the Sweep packet serial sample only moves forward and stops at 9600.
We've run into this with the Arduino Uno and Sabertooth 2X5. There is a startup transient
on the Uno that the Sabertooth sees as a very fast autobaud character. As a result it
autobauds at its maximum baud rate, 38400. Changing the Serial.begin(9600); line to
Serial.begin(38400); got things running for us.